Here are a few of the tools I live by personally and professionally (forgive if some have been listed, I was not able to find these prior here):

Crap Cleaner - A very powerful system cleanup utilityGIMP - The GNU Image Manipulation Program (Open Source alternative to Photoshop/Paint Shop Pro/Flash)TrueCrypt - Open Source Disk Encryption utilityFree AVG - Free version of a very powerful yet low resource usage Anti-Virus utility. Award winning and really a must have for gamersAUSLogics Disk DeFrag - A decent Disk Defragmenter alternative to the standard built-in one with WindowsHDTune - A free version of the powerful HDD benchmark utility (only able to monitor read speeds as the full version grants access to write benchmarks and a few other tools)